This invention relates to the purification of fluids, and more particularly to the purification of a fluid by the removal of small amounts of fluid impurities from the fluid by adsorption.
The purification of fluids by the removal of small concentrations of fluid impurities can be accomplished by several physical and chemical techniques, including distillation, chemical reaction and adsorption. In some cases it is desirable to use one of these procedures while in other cases it is preferable to use another. The separation of argon from air and subsequent purification of the separated argon illustrates a case in point.
Crude argon produced by the cryogenic distillation of air by conventional techniques generally contains 1-5% by volume oxygen and up to about 1% by volume nitrogen. If it is desired to produce higher purity argon, for example argon containing less than about 10 parts per million (ppm) each of oxygen and nitrogen, the oxygen and nitrogen are removed from the argon stream by one or more of the available physical or chemical techniques. According to one technique oxygen and nitrogen are removed from the crude argon stream by further distilling the crude argon stream. This method of purification is capital intensive because the boiling points of oxygen and argon are only a few degrees apart; consequently a very high column with a large number of trays is required to reduce the oxygen content to the parts per million (ppm) range.
Another technique that has been employed is reacting the oxygen in the argon stream with excess hydrogen over a suitable catalyst at relatively high temperatures, and subsequently removing the excess hydrogen and nitrogen by cryogenic distillation. Removing oxygen by this technique requires a significant quantity of energy, however, since the gas stream must be heated to a relatively high reaction temperature and later cooled to cryogenic temperatures to distill off the excess hydrogen and the nitrogen present in the gas stream. Furthermore, the high purity hydrogen required for the oxidation is not always available at locations where it is desired to operate such argon purification plants.
Argon has been separated from nitrogen and oxygen by pressure swing adsorption (PSA) at ambient temperatures. U.S. Pat. Nos. 4,144,038 and 4,477,265 disclose adsorption of oxygen and nitrogen from an argon-rich feedstock withdrawn from the rectification column of a cryogenic air separation plant. These processes suffer from low yield and low purity of the argon product.
Recently, several cryogenic adsorption processes for the removal of both oxygen and nitrogen from argon have been developed. The removal of oxygen and nitrogen from argon at below ambient temperatures (173 to 273 K.) by PSA and a combination of PSA and temperature swing adsorption (TSA) is described in: German Patent 2,826,913 (which discloses the use of a mixture of 4A and 5A zeolites as adsorbents); Japanese Patent Kokai 59/064,510 (which uses a mixture of mordenite and faujasite as adsorbent); and Japanese Patent Kokai 58/187,775 (which uses type A zeolite as adsorbent). In the TSA embodiments of these disclosures adsorption capacities are fairly low resulting in very large bed requirements, and in the PSA embodiments high purity argon product yields are low.
The removal of oxygen alone or the removal of both oxygen and nitrogen from argon at cryogenic temperatures (90 to 173 K.) by adsorption using 4A type sieve is described in Japanese Patent Kokai 62/065,913; by Fedorov et al. in Khim. Neft. Mashinostr. (Vol 6, page 14, 1990); and by Kovalev et al. in Energomashinostroenie (Vol 10, page 21, 1987). The disadvantage of this technique is that when both nitrogen and oxygen are present in the gas stream being treated, nitrogen interferes with the adsorption of oxygen on the 4A sieve. Consequently, very large beds are required for complete oxygen removal. If nitrogen is removed by cryogenic distillation prior to adsorption, 4A zeolite sieve is effective for oxygen removal; however, this increases the cost of argon purification.
The adsorption of gases onto an adsorbent is an exothermic process. Accordingly, the temperature of an adsorbent will rise during the course of an adsorption process because of the heat given off during the adsorption. Furthermore, the quantity of heat given off is directly proportional to the concentration in the gas mixture of the component that is being adsorbed: the more gas impurity to be adsorbed from a gas mixture the greater the amount of heat given off during the adsorption step and the greater the temperature rise.
In most gas adsorption processes the adsorption efficiency is inversely proportional to the temperature at which the adsorption is conducted. The ability of an adsorbent to adsorb a given gas generally diminishes as the temperature of the adsorption bed increases. Because of this it is usually desirable to conduct the adsorption at a low temperature, and to minimize any increase in bed temperature as the adsorption proceeds.
The problem of temperature rise can be particularly acute when, for the purpose of maintaining product purity specifications it is necessary to conduct an adsorption process at just above the dew point of the gas mixture, and even a small increase in bed temperature will cause the product to fail to meet purity requirements. In such cases it is often necessary to reduce the concentration of the impurity to be adsorbed as much as possible by other techniques prior to the adsorption procedure and to apply cooling to the bed to maintain it at constant temperature during adsorption. The difficulties caused by the phenomenon of bed temperature rise during cryogenic adsorption of crude argon are dealt with in various ways, two of which are illustrated in the following patents.
U.S. Pat. No. 3,928,004, issued to Bligh et al. on Dec. 23, 1975, discloses a process for the purification of crude argon gas by passing the gas through a bed of molecular sieve at a temperature near the dew point of the argon. Before the adsorption step nitrogen is removed by distillation. After the bed is regenerated it is cooled to as close as possible to the dew point of the argon, as it is necessary to conduct the gas adsorption at such low temperatures to produce argon of the desired purity. Since the bed warms up due to the heat of adsorption, the purity of the effluent from the bed falls off as the adsorption step progresses.
U.S. Pat. No. 5,159,816, issued to Kovak et al. on Nov. 3, 1992, discloses the production of high purity argon (containing less than 5 ppm each of nitrogen and oxygen) by cryogenic adsorption by passing gaseous crude vapor argon feed first through a bed of adsorbent which preferentially adsorbs nitrogen and then through a bed of adsorbent which preferentially adsorbs oxygen. The process can be carried out without the need of refrigeration by maintaining a low gas space velocity through the beds and by limiting the content of oxygen and nitrogen in the crude vapor argon feed to the adsorption system to no more than 0.8 mole percent and 0.5 mole percent, respectively.
The temperature rise during adsorption can be significant. For example, the temperature rise experienced during the adsorption of oxygen from an argon gas stream in a bed of 4A zeolite at cryogenic temperatures is as much as 60.degree. C. when the feed gas mixture contains up to 3.5% by volume of oxygen. Since the oxygen capacity of 4A zeolite at cryogenic temperatures diminishes rapidly as the temperature rises, vapor phase adsorption of oxygen from crude argon containing more than about 1.0% by volume oxygen in a bed of 4A zeolite is not a suitable technique when high purity argon product is sought.
Because of the importance of substantially pure argon, for instance in the electronics field, economical high efficiency and high yield processes for removing both of these impurities from an argon stream are constantly sought. The present invention provides such a process.